The present invention relates generally to functionally-graded materials and more particularly to composite laminates and other constructions, in which each layer differs in a principle axis of anisotropy of less than 45xc2x0 relative to its preceding layer, the construction exhibiting improved resistance to failure under indentation perpendicular to the layer arrangement, and functionally-graded articles designed to withstand sliding contact stress.
Layered structures define a wide variety of construction arrangements, including lightweight laminated composite articles. Laminated composites typically are defined by a continuous, essentially planar array of continuous fibers embedded in a matrix defining a lamina or ply, a plurality of plies forming a laminate composite. These composite articles can be arranged to meet various in-plane stiffness or strength requirements by appropriate stacking of similar orthotropic plies of dissimilar orientation. Arrangements of this type have been analyzed in terms of their ability to provide strength in a direction in alignment with the planes of the construction.
U.S. Pat. No. 5,064,439 (Chang) describes a load-bearing orthopedic prosthetic device for human implantation, such as a hip stem prosthesis, made of layered plies each including parallel-oriented continuous filament fibers. The arrangement is such that load is born primarily in the direction of the planes of the plies. The plies are arranged such that at or near the surfaces of the devices, plies are oriented with fibers running longitudinally. That is, at or near the surfaces of the devices fibers are oriented in the longitudinal direction of the bone replaced by the prosthesisxe2x80x94the primary direction of load. The plies are stacked with fibers offset at 5xc2x0-40xc2x0 from the longitudinal axis between the surface layers. Chang tests failure load properties of a wide variety of balanced and unbalanced stacked arrays, noting that completely unbalanced fiber orientations should not be used because of substantially lower strengths that purportedly result. Optimal fiber orientations, according to Chang, include [O2/xc2x17.5/xc2x115/xc2x122.5/xc2x130]s; [O2/(xc2x120)4]s; and [O2/xc2x120/xc2x130/xc2x120/xc2x130]s, where fiber orientations are given in degrees and ordered from an exterior surface for each layer into the middle plane, and are symmetric about the middle plane, with xe2x80x9cxc2x1xe2x80x9d indicating one layer at a positive angle and one at a negative angle. Table III of Chang lists other fiber orientations tested, both balanced and unbalanced.
U.S. Pat. No. 5,591,233 (Kelman) describes a metal/composite hybrid orthopedic implant such as a hip prosthesis. The composite portion is comprised of filaments non-linearly disposed to produce a structure of variable modulus along its length.
U.S. Pat. Nos. 5,624,386 (Tailor) and U.S. Pat. No. 5,522,904 (Moran) also describe composite laminate articles for orthopedic braces and implants, respectively, including multiple fiber layers oriented in at least two directions.
U.S. Pat. No. 5,437,450 (Akatsuka) describes a golf club shaft composed of an inner tubular layer of a carbon fiber-reinforced plastic having a fiber orientation angle of 40-50xc2x0 with respect to the longitudinal axis of the shaft, and an outer tubular layer of carbon fiber reinforced plastic provided around the inner layer and having a fiber orientation angle of 5-30xc2x0. The resultant shaft purportedly is lightweight but exhibits high mechanical strengths suitable for use with a large metal golf club head.
In the field of biomedical implants, mechanical machinery and related fields, there are many situations in which one particular device is designed to repeatedly engage in sliding contact with another device. For example, in the case of biomedical implants such as hip, knee, shoulder, and finger joint protheses, pins, screws, and dental implants, and mechanical components such as gears and ball bearings, the resistance of the article to sliding wear is critical to the success for long-term use. One approach to producing wear-resistant articles designed to withstand sliding contact has been to employ graded materials. The use of graded materials has, in some cases, led to better stress distribution from a prosthetic implant to an attached or adjacent bone (J. B. Park, Biomaterials: An Introduction, New York, Plenum Press). That is, it is known that the modulus of a prosthetic implant, at a surface that is designed to engage bone, should match as closely as possible the modulus of the bone, and the use of graded materials has in some cases allowed such matching.
Several of the above and other documents represent composite, fibrous laminate articles including plies that are offset with respect to fiber direction to affect certain strength and stiffness requirements. However, study and development of layered articles tailored to provide optimal impact resistance in a direction perpendicular to the plane of the stacked array are inadequate. Loads applied perpendicular to the plane of stacked arrays can cause stress concentrations at interfaces between ply groups of dissimilar anisotropic orientation. Spherical indentation studies of cross-ply laminates show that stress concentrations initiate transverse cracks, delamination, and/or splitting of fibers which lowers the indentation strength and can result in eventual failure of the structure.
Additionally, techniques for improving the wear resistance of articles designed to withstand sliding contact are known. However, the problem of wear of such components has severely limited the success of long-term implantation of many medical prosthesis. This has led to patient discomfort and higher medical costs. The exacting demands on traditional and high-technology devices of this sort require new, better wear-resistant composites.
Accordingly, it is an object of the present invention to provide layered constructions that exhibit improved structural resistance to indentation or impact in a direction perpendicular to the planar orientation of the structure. It is yet another object to provide better wear-resistant articles constructed to withstand sliding contact.
According to one aspect of the invention a series of constructions are provided, which can be articles. In one embodiment, the construction is defined by a man-made, functionally-graded material having Young""s modulus that increases as a function of depth from the surface of the material. The construction exhibits Young""s modulus at the surface of at least about 1 GPa. According to another embodiment the construction is a man-made functionally-graded material having Young""s modulus that increases as a function of depth from the surface, wherein the construction has a Poisson ratio of at least about 0.2.
In another embodiment a construction is provided that is a man-made, functionally-graded material having Young""s modulus that increases as a function of depth from the surface at a rate that decreases.
Another aspect of the invention involves a method of making a material. The method involves providing a first material and a second material, and allowing the second material in a fluid state to infiltrate voids at a surface and into the first material under conditions and for a period of time sufficient to create a composite article. The composite article is graded in content from relatively lower first material content at the surface to complete first material content within the functionally-graded article.
In another aspect the invention provides an article for resisting indentation against its surface. The article includes a stacked array of at least five layer units each having a principle axis of anisotropy. Each unit has an adjacent unit, and the principle axis of anisotropy of each unit is offset from that of a unit to which it is adjacent by less than 45xc2x0. The article forms part of a construction constructed and positioned to withstand indentation having a component normal to a tangent of the surface of the stacked array.
In another embodiment the invention provides an article, having a surface, that is made of at least five layer units each having x, y, and z axes. The layer units have, individually, prior to assembly, x-axis mechanical properties, y-axis mechanical properties, and z-axis mechanical properties that are essentially identical to each other. That is, the x-axis mechanical properties of each layer are identical to the x-axis mechanical properties of each adjacent layer. The layer units form a stacked array, having a surface, that has Young""s modulus that increases as a function of depth from the surface. The article forms part of a construction constructed and positioned to withstand contact damage having a component normal to a tangent of the surface.
In another embodiment the invention provides an article for resisting contact damage or indentation against its surface. The article is a stacked array of at least five layer units. Each layer unit is isotropic overall in layer directions and has local anisotropy in at least one direction. Each unit has an adjacent unit that differs in a mechanical property, and the article forms part of a construction constructed and positioned to a stand indentation having a component normal to a tangent of the surface of the stacked array.
According to another aspect the invention provides methods. One method involves forming an article having a surface constructed and positioned to withstand indentation having a component normal to a tangent of the surface and having increased stiffness. The article is formed by arranging an array of at least five layer units, each having a principle axis of anisotropy that differs from that of an adjacent unit by less than 45xc2x0.
In another embodiment, the invention provides a method involving subjecting a stacked array of at least five layer units, having a surface, to indentation having a component normal to a tangent of the surface of at least ⅕ the tangential force tolerable without failure of the construction. The stacked array is an array of at least five layer units each having anisotropy in at least one direction. Each unit has an adjacent unit that differs in a mechanical property or offset in principle axis of anisotropy by less than 45xc2x0.
In another aspect the invention provides an article having a surface where the surface is constructed and arranged to be positioned to withstand a sliding contact stress. In this aspect the surface also is constructed to be a functionally-graded material during use.